In today's television receivers and computer TV cards it is necessary to separate synchronisation signals (or in short: sync signals) to achieve a proper representation of the video image on the display, e.g. a cathode ray tube.
Even though digital system solutions will become increasingly predominant in future television technology, analogue source signals will still exist for many years in the future. Examples include the terrestrial reception of video signals, which is still widespread to date, and the analogue recording methods, e.g. according to the VHS standard in the case of video recorders. Such analogue signal sources represent critical signal sources for digital systems, and their signal processing requires special measures. The situation is the same for future multimedia terminals as long as they are equipped with an analogue video input. Due to the existence of large quantities of analogue video material, e.g. VHS tape libraries, it is unlikely that the use of analogue video signals could disappear in the near future.
Conventionally, a sync slicer is used for sync separation. The sync slicer is combined with a PLL (Phase Locked Loop) for “smoothing” the extracted sync information. The PLL technology is mature but has well known limitations due to two conflicting requirements: On the one hand the PLL must have a low-pass characteristic to suppress disturbances in the sync signal detection caused by noise. On the other hand, tape recorders, in particular camcorders, output the video signal on a variable time base due to mechanical tolerances. The time base variations also appear as disturbances of the sync signal to the PLL. However, this kind of disturbance has to be passed through without attenuation because any alterations of the time base would create horizontal instabilities of the displayed video picture. In other words: The PLL has to suppress noise on the one hand and has to pass time-base variations on the other hand. Fortunately, the two effects are distinguishable by their frequency. Time-base variations are a low frequency effect above 1 kHz. As the PLL is always a second order loop, corner frequencies and stability are selected in every new designed system for best compromise.
Television receivers with digital signal processing (e.g. in the case of the 100 Hz technology) have been operating, as a rule, with clock systems, which are synchronized with the respective input signal. Since the input signal is the analogue CVBS signal, either the horizontal sync pulse (line-locked clock) or, alternatively, the colour subcarriers or colour synchronizing pulses (burst) (colour subcarrier-locked clock) are frequently used as reference point for the synchronization. The sync separation in the video lines has usually been carried out to date by means of analogue methods using so-called sync separator stages and a PLL filter stage connected downstream. In television receivers with digital signal processing, a PLL filter stage, which is a digital realization of the known analogue sync signal processing is commonly used. The filter stage is then a digital PLL (Phase-Locked Loop). Examples of such digital PLL circuits are the circuits SAA 7111 from Philips, HMP 8112 from Harris and Digit 3000 from Micronas. The principal problem with such digital PLL circuits is that the known instabilities in the picture occur when the input signal present is an analogue video signal picked off from an analogue video recorder, which is currently operating in the search mode (fast forward or reverse run). Many users of analogue video recorders are sufficiently acquainted with such instabilities. Specifically, disturbing horizontal stripes appear in the picture when the video recorder is operating in the search mode. These disturbing stripes originate from the fact that in the search mode, the video heads no longer run on a single slanted track but rather sweep across two or more slanted tracks, depending on the search speed. During the transition from one slanted track to the next, abrupt sudden phase changes arise with regard to the occurrence of the sync pulses of the video lines. These sudden phase changes are actually governed by the geometry in magnetic tape recording in accordance with the slanted track method. The sudden phase changes are therefore determined by the system and, in addition, virtually unavoidable.
Irregular occurrence of line sync pulses also arises, however, in the case of video signals generated by camcorders. In this case, the instabilities that occur are, as a rule, more severe than in the case of a normal video recorder, because the regulation of the head-drum speed is subject to greater fluctuations on account of the larger component tolerances.
EP-A 0 266 147 discloses a digital PLL circuit for a television receiver. In the case of this digital PLL circuit, in order to avoid-the abovementioned problem in the search operating mode in video recorders, a switching unit is provided which drastically shortens the time constant of the phase-locked loop in the event of identification of a sudden phase change caused by the head changeover at the end of a slanted track, with the result that the region of instability in the picture is reduced in size. The disadvantage of this solution is that the reduction of the time constant of the phase-locked loop provided by this solution means that noise components in the video signal are able to be suppressed less well and disturbing lines still remain visible, even though to a lesser extent than when the time constant is larger.
In EP-A 0 899 945 a method is for obtaining line synchronization information is described. According to the known method a video line is convoluted with an idealized horizontal synchronization pulse. The result of the convolutions processed in an open loop system replacing the PLL. The open loop system is realized by a linear regression to extrapolate the best guess of a current sync pulse using past sync pulses.
Convolving, or convolution, is a well-known term meaning the integral of one function multiplied by another function, which is shifted in time, see for example in “New IEEE Standard Dictionary of Electrical and Electronics Terms”, 1993.
Using the known systems as a starting point it is desirable to have a method providing an even better performance with regard to horizontal synchronization of the video lines.